1. Field of the Invention
The present invention relates in general to sensing devices and more specifically to such devices that employ a pyroelectric material for sensing.
2. Description of the Prior Art
Pyroelectric sensing devices have recently become well known in the art. One of the major problems that is inherent in pyroelectric sensing devices is that they must be temperature compensated in order to minimize erroneous readings due to ambient temperature changes in the vicinity of the device. A recently issued patent to Rossin, U.S. Pat. No. 3,839,640 discloses a detector developed to overcome the problem of ambient temperature changes by employing a pyroelectric film that is coated on its front surface with a pair of conductive strips and on its back surface with a U-shaped conductive strip in back-to-back relationship with the front strips.
The Rossin detector includes a front mirror reflector so that radiation desired to be detected will be focused on only one of the front conductive strips. The front conductive strips are connected to an electronic sensing circuit in such fashion that when only one of the front conductive strips is heated a sensing indication will be provided, and when both front conductive strips are heated by a change in ambient temperature no sensing indication results. Accordingly, the Rossin detector achieves the goal of temperature compensation. However, because of such compensation the sensitivity of the Rossin detector is substantially lower than that of a noncompensated detector. The reduced sensitivity results because the conductive strips of the Rossin detector form two capacitors that are electrically connected in parallel. Since voltage provided the sensing circuit due to the heating of the conductive strips is inversely proportional to the amount of circuit capacitance, the parallel capacitors of Rossin provide less sensing voltage than would be provided if only a single capacitor were used.